1. Field of the Invention
The present invention relates to an apparatus and method for plasma processing. More particularly, the invention relates to methods and apparatus for minimizing process sensitivity to chamber conditions during etching processes.
2. Background of the Related Art
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited and removed from a substrate during the fabrication process. Substrate etching methods and apparatus used in device manufacture for the purpose of removing material from the substrate are well known. Typical etching techniques include wet and dry etching. However, wet etching is typically limited to fabrication of components with lateral dimensions of a micron or greater. Solid state devices and integrated circuits are now routinely fabricated with sub-micron or even nanometer scale components. Therefore, dry etching is now the preferred etching method.
One dry etching technique is commonly known as plasma enhanced etching (xe2x80x9cplasma etchingxe2x80x9d). Plasma etching is very well suited for the manufacture of nanometer sized devices. A conventional plasma etching reactor includes a reactor chamber and an apparatus for producing a plasma within the reactor chamber. The plasma may be produced inductively, e.g., using an inductive RF coil, and/or capacitively, e.g., using a parallel plate glow discharge reactor. Typically, the plasma is struck and maintained both capacitively and inductively.
In general, plasma etching involves positioning a mask on an upper substrate surface to define an exposed portion of the substrate to be etched. The substrate, or batch of substrates, is then placed in the reactor chamber. Etching gases are introduced into the reactor chamber and a plasma is struck. During processing, the reactive species in the plasma etch the exposed portion of the metal, dielectric, or semiconductive material by contacting the exposed portion of the substrate.
At the molecular level, the etch process is a reaction between the reactive species in the plasma and the exposed surface layers of the substrate. The species include free radicals, ions and other particles. Although the reaction between the substrate and the free radicals is essentially chemical in nature, it is greatly enhanced with the ion bombardment which contributes to the etching and provides activation energy to the surface reaction. The reaction between the plasma and a substrate yields etch byproducts, i.e., small volatile molecules that desorb from the surface and subsequently are diffused into the reactor chamber. Most of the volatile byproducts are then pumped out of the reactor chamber.
Etching of a single layer of material generally comprises two primary steps: a main etch process and an overetch process. The main etch process removes the bulk of the material from the exposed substrate surface to form the desired feature. The overetch process is needed in order to remove residual material from the substrate while avoiding undercutting (i.e., isotropic etching) and excessive loss of selectivity between interfacing layers, such as a polysilicon/oxide interface. The chemistry and process parameters for each step are selected to achieve anisotropy, constant, and preferably high, etching rates, uniformity, high selectivity, and reproducibility.
Successful etching requires a controlled process to ensure uniform etching at a controlled and constant rate with respect to each individual substrate as well as from one substrate to the next. Uncontrolled changes in etching rates can lead to changes in device geometries and dimensions. Preferably, an etch rate change for a particular substrate and from one substrate to the next is less than about 10%.
Process stability is affected by various methods and techniques commonly used in the industry. For example, a substantial change in the etch rate is observed after a cleaning process. Cleaning processes are periodically necessary to remove deposits of byproducts which form on the internal chamber surfaces during processing. Most volatile byproducts formed during etching are pumped out of the chamber. However, the byproducts can often react with various gas components used in silicon etching with a halogen plasma, such as oxygen additives, to form less volatile byproducts. In other cases, the byproducts themselves can be less volatile, depending on the material being etched and the etch chemistry. These less volatile species can deposit onto the chamber walls and other exposed surfaces enclosed within the chamber. Over time, the deposits can delaminate and flake off of the chamber surfaces creating a major source of particulate contamination. The particulate often become lodged in the mask or on the substrate surface and produce defective devices. As the size of the etched features become smaller, the effects of particulate become more pronounced.
Thus, in order to control the contamination buildup, the chamber surfaces are cleaned periodically. One method of cleaning a chamber, known as dry cleaning, involves placing a dummy substrate in the chamber and then igniting a plasma The plasma chemistry is selected to react both chemically and physically with the deposits on the chamber, thereby causing the deposits to form byproducts which can be pumped out of the chamber. However, a problem associated with chamber cleaning, is that the etching rates of the subsequent processes are adversely affected. In typical silicon plasma etching, etch rate drops in excess of 33% have been experienced following a chamber cleaning run. Changes in the etch rate are undesirable because of the resulting loss of process reproducibility, or repeatability. Because etching processes are often timed according to pre-programmed recipes, the fluctuation in etch rates results in over-etched or under-etched substrates. Consequently, repeatability between substrates is lost.
In order to minimize the detrimental effects of etch rate variation after a cleaning process, current practice employs a seasoning cycle during which the chamber is conditioned following a cleaning run. Seasoning refers to the operation of the chamber to allow deposition of a film on the chamber surfaces. During a recovery period a seasoning coating is allowed to form on the chamber surfaces by striking a plasma in the chamber and depositing a film on the internal exposed surfaces in the chamber. The chamber seasoning is continued until the pre-clean etch rate is completely recovered. The recovery period is time consuming and non-productive because no substrates are processed. Thus, the throughput of the system is significantly reduced.
Another problem related to process stability occurs when high concentrations of corrosive chemicals are used. For example, a problem exists in cases where fluorine is the major etchant, such as in planarization and recess-etching. Fluorine is a highly corrosive etchant that attacks the chamber surfaces causing a change in the topography of the surface and/or resulting in deposition of a byproduct, such as AlFx in the case of an Al2O2 chamber surface. Unlike deposition of SiOx which can be removed by non-intrusive cleaning methods such as plasma cleaning, as described above, the effects of fluorine etching must be treated by opening the chamber and refinishing the chamber surfaces. Thus, throughput is substantially affected. Additionally, following the cleaning process, fluctuations in the etch rate are observed which inhibit etch rate stability.
Sensitivity to chamber conditions was also observed by the inventors in situations other than post-cleaning. For example, when the same chamber is used to run different applications involving alternating chemistry, varying etch rates are observed. Thus, processes alternating between fluorine-based chemistry and non-fluorine-based chemistry experience volatility in etch rates between each cycle. As an example, a chamber may first be used to process a number of substrates during a fluorine-based process such as a hardmask open or recess etching. Subsequently, the chamber may be used to run a second process not involving the use of fluorine, such as capacitor etching. For reasons heretofore not understood, the etch rates under such conditions experience fluctuation leading to loss of process repeatability.
Therefore, there is a need for reducing the process sensitivity of a processing chamber to the chamber surface conditions.
The present invention generally provides a method and apparatus of minimizing etch process sensitivity to chamber conditions. The etch process sensitivity can be controlled by various methods including changing the etch chemistry, process parameters and/or changing the chamber materials.
In one aspect of the invention, a composition of one or more etchants is selected to optimize the etch performance and reduce deposition on chamber surfaces. The one or more etchants are selected to minimize buildup on the chamber surfaces, thereby controlling the chamber surface condition to minimize changes in etch rates due to differing recombination rates of free radicals with different surface conditions, thereby allowing etch repeatability. In one embodiment, at least one etchant is introduced into the chamber and a plasma is struck to form at least a first free radical density from the first etchant. The etchant is selected to minimize the rate at which the free radical density reacts to produce deposition on the internal surfaces of the chamber and wherein the recombination rate of the free radical density on the deposition formed on the internal surfaces is substantially different than the recombination rate of the free radical density on the internal surface. In another embodiment, chlorine and bromine are introduced into a chamber at a first and a second flow rate, respectively; wherein the first flow rate is higher than the second flow rate. Preferably, the ratio of chlorine to bromine is greater than about 3:1.
In another aspect of the invention, a first and second etchant are flowed into an etch chamber wherein the first etchant is adapted to clean the internal surfaces of the chamber and prevent the deposition of byproducts thereon while simultaneously etching a substrate. A substantially constant chamber surface profile is maintained in order to maintain a substantially constant recombination rate of plasma constituents with the chamber surfaces. Preferably, the first etchant is a fluorine-containing fluid and the second etchant is preferably a halogen, such as chlorine or bromine.
In yet another aspect of the invention an etch chemistry is selected to minimize corrosion of a chamber surface. A first etchant having corrosive properties is diluted with a second etchant wherein the second etchant is preferably non-corrosive or less corrosive than the first etchant. The first etchant is a fluid exhibiting corrosive effects when brought into contact with an internal chamber surface. The second etchant is combined with the first etchant to reduce the corrosive effects of the first etchant on chamber surfaces. The resulting mixture preferably exhibits reduced corrosive effects on chamber surfaces and allows successive processing of multiple substrates under substantially similar chamber surface conditions. In one embodiment, the first etchant is a fluorine-containing fluid such as CF4, SF6 or NF3. The second etchant is preferably a chlorine-containing fluid such as Cl2.
In yet another aspect of the invention, a chamber pressure is controlled to reduce the sensitivity of an etch process to the chamber surface conditions. The chamber pressure is controlled during an overetch process to increase the ratio of ions to free radicals, thereby increasing ion-assisted etching and reducing pure free radical etching. In one embodiment, the pressure during an overetch of a polysilicon layer is preferably less than about 30 mTorr. The etch process is preferably bromine-based or chlorine-based.
In yet another aspect of the invention, the chamber pressure and the concentration of oxygen in the chamber is controlled to reduce the sensitivity of an etch process to the chamber surface conditions. The chamber pressure is controlled during an overetch process to increase the ratio of ions to free radicals, thereby increasing ion-assisted etching and reducing free radical etching. The oxygen concentration is controlled to reduce oxidation of a substrate surface being etched. In one embodiment, the pressure during an overetch of a polysilicon layer is preferably less than about 10 mTorr and the oxygen concentration is less than about 25% of the total flow volume of gases into the chamber. The etch process is preferably bromine-based or chlorine-based.
In yet another aspect of the invention, the chamber pressure, the concentration of oxygen in the chamber, and the source power is controlled to reduce the sensitivity of an etch process to the chamber surface conditions. The chamber pressure is controlled to increase the ratio of ions to free radicals, thereby increasing ion-assisted etching and reducing free radical etching. The oxygen concentration is controlled to reduce oxidation of a substrate surface being etched. The source power delivered to an inductive coil is adjusted to control substrate surface oxidation and etching. In one embodiment, the pressure during an overetch process of a polysilicon layer is preferably less than about 10 mTorr, the oxygen concentration is less than about 25% of the total flow volume of gases into the chamber and the source power is between about 100-500W. Preferably, the oxygen concentration and the source power are inversely adjusted, such that when the oxygen concentration is increased, the source power is decreased and vice versa. The etch process is preferably bromine-based or chlorine-based.
In still another aspect of the invention, the chamber materials are selected to reduce the sensitivity of an etch process to the chamber surface conditions. The materials are selected according to the recombination rate of free radicals therewith. Preferably, the recombination rate of the free radicals with the selected materials is substantially equal to the recombination rate of the free radicals with the byproducts formed during the etch process and deposited on the chamber surfaces. Thus, the etch rate is substantially constant over time even where deposits are formed on the chamber surfaces. In one embodiment, the chamber components exposed to the processing environment of a silicon etch chamber, such as the chamber body and the chamber dome/lid, substantially comprise quartz. The chamber components may be constructed of quartz or, alternatively, may be lined with quartz.
In still another aspect of the invention, the chamber temperature is controlled to reduce the sensitivity of an etch process to the chamber surface conditions. The temperature is preferably sufficiently high to reduce deposition on exposed chamber surfaces. In one embodiment, a polysilicon layer is etched at a chamber temperature of at least about 200xc2x0 C.